1. Field of the Invention
The invention relates to an electric motor.
2. Discussion of Background
Conventionally, there has been an electric motor that includes a so-called “embedded magnet rotor” in which permanent magnets are embedded in a rotor core so that the permanent magnets are fixed to the rotor core (for example, Japanese Patent Application Publication No. 2010-233346 (JP 2010-233346 A)). Generally, in such a permanent magnet electric motor having permanent magnets in a rotor, magnetic flux produced by the permanent magnets is constant. Therefore, an induced voltage (a counter-electromotive voltage) generated in a coil of a stator is increased in proportion to a rotation speed of the rotor. Then, once the induced voltage reaches an upper limit of a power-supply voltage, it is no longer possible to rotate the rotor at a higher speed. Thus, a design may be employed, which restricts an amount of magnetic flux produced by the permanent magnets to an amount at which the rotor is allowed to rotate at a sufficiently high speed. However, in this case, there is a possibility that torque will be insufficient in a low-speed rotation region.
In recent years, an electric motor has been proposed, which includes a housing (a magnetic field yoke) having a tubular portion and a top plate portion provided at an open end of the tubular portion, a stator fixed to an inner periphery of the tubular portion, and a rotor arranged radially inward of the stator, and in which a field coil that is wound along the circumferential direction of the electric motor is fixed to the top plate portion such that the field coil faces the rotor in the axial direction of the electric motor (for example, US 2008/0036331 A1). The electric motor is structured such that magnetic flux produced by the field coil passes through the top plate portion of the housing, the rotor, the stator, and the tubular portion of the housing. Further, by changing a direction of magnetic flux that is produced by the field coil, it is possible to adjust an amount of magnetic flux that is transmitted between the stator and the rotor (a magnetic flux density on an outer periphery of a rotor core). By producing, in the field coil, magnetic flux that reduces an amount of magnetic flux that is transmitted between the stator and the rotor, it becomes possible to suppress an increase in an induced voltage and allow the rotor to rotate at a high speed. In addition, by producing, in the field coil, magnetic flux that increases the amount of the magnetic flux, it becomes possible to generate high torque in a low-speed rotation region.
In the structure described in US 2008/0036331 A1, the housing serves as a magnetic path for magnetic flux produced by the field coil. Therefore, if a thickness of the housing is reduced, magnetic resistance of the magnetic path increases. If magnetic resistance becomes excessive, it may no longer be possible to sufficiently adjust an amount of magnetic flux that is transmitted between the stator and the rotor on the basis of the magnetic flux produced by the field coil. Therefore, it is difficult to reduce the size of an electric motor.